This invention relates generally to electronic trip units, and more particularly, this invention relates to a method and circuit breaker for determining contact wear of the circuit breaker at an electronic trip unit.
Electronic trip units are well known. An electronic trip unit typically comprises voltage and current sensors which provide analog signals indicative of the power line signals. The analog signals are converted by an A/D (analog/digital) converter to digital signals which are processed by a microcontroller. The trip unit further includes RAM (random access memory), ROM (read only memory) and EEPROM (electronic erasable programmable read only memory) all of which interface with the microcontroller. The ROM includes trip unit application code, e.g., main functionality firmware, including initializing parameters, and boot code. The EEPROM includes operational parameters for the application code. An output of the electronic trip unit actuates a circuit breaker. The circuit breaker typically includes a pair of contacts which allows circuit current to pass from one contact member to another contact member. When the contacts open, circuit current is prevented from flowing from one contact member to the other and therefore, circuit current is prevented from flowing to a load which is connected to the breaker.
Circuit breaker contact wear is a frequently occurring yet difficult to measure or predict problem because it is affected by a variety of factors. Contact wear is affected by the cumulative energy dissipated through arcing as breakers are opened. However, a single severe over-current fault can destroy contacts more quickly than several smaller faults, even though the smaller faults may add up to the same total energy dissipated. Merely counting the number of faults experienced by a breaker may not give an accurate assessment of contact wear. Contacts are not generally easily inspected without costly disassembly and power down. However, if not detected, contact wear may result in loss of power.
The only current solution to this is defensive preventative maintenance, whether required or not. Industrial plants, however, typically have fewer maintenance personnel than is necessary to adequately provide this preventative maintenance. Additionally, in many instances, there are no clear guidelines for replacing circuit breakers as a function of actual service duty. Power management systems may connect to low voltage circuit breaker trip units and provide breaker status, but they do not provide a systematic approach to measuring and reporting on breaker health on a condition basis. Scheduled, preventative maintenance is the only option. Thus, there are correspondingly no reliable methods for identifying breaker health remotely in low voltage circuit breakers.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a method of detecting contact wear of a pair of separable contacts in an electronic trip unit of a circuit breaker. The method comprises issuing a trip or open signal to initiate a separation of said pair of separable contacts; detecting the separation of said pair of separable contacts; sensing current through said pair of contacts to provide a sensed current signal indicative thereof; and integrating the sensed current signal after separation of said pair of separable contacts is detected to result in a first measurement proportional to contact wear of said contacts.
Similarly, a breaker assembly comprising an electronic trip unit and a circuit breaker having at least one pair of separable contacts, also includes means for detecting separation of said at least one pair of separable contacts; a current sensor positioned for sensing current through said at least one pair of separable contacts to provide a sensed current signal indicative thereof; and, a signal processor responsive to said first sensed current signal, and having memory for storing signals including program signals defining an executable program, wherein said processor integrates said first sensed current signal to after separation of said at least one pair of separable contacts is detected to result in a first measurement proportional to contact wear of said contacts.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.